It is important first to consider whether the child experiencing nightmares has been through some sort of trauma. In this case, you will want to find professional help regardless, as this child is most likely reliving the trauma in his or her dreams. Dreams are a way for the subconscious to work through various events, which is fine as long as those events have not been terrifying. Be sure to address the trauma as soon as possible, to ensure the terror in the night does not become a regular occurrence.
Children who have not been through trauma can still experience nightmares and will need your help as their parent to process through them. If your child gets out of bed be sure to bring them back to their own bed. Show them there is nothing to fear by being in their own room. Be careful not to dismiss their fears entirely, but rather, be a sport and check under the bed, open up the closet doors, as well as any other possible hiding places. You child will appreciate the peace of mind that comes with having checked these possible scary spots without having felt as though their feelings have been dismissed.
Once the perimeter has been cleared, tuck them right back into bed, letting them know you will stay for awhile to help them get through the rough moment. Ask them what happened in their dream, asking them to explain step by step. Often what happens in their head will be much scarier if it is not voiced aloud. Speaking the fear into existence will allow your child to rationalize out the reality of their concerns on their own. If they need a little help with that piece, well that is what you are there for.
Teach your child to defend themselves against the offenders in their dreams. Should someone be chasing your child, tell your child to stop in their tracks, turn around, and face their attacker. Ask them how they might win an argument or a fight with this attacker. If they are unsure, offer your own ideas, without judgement. Is your child being chased by a vampire bat? Share with them the idea of catching the bat with a butterfly net, complete with garlic hanging on the rims, as vampires could not possibly survive being exposed to garlic. You will be surprised of what years of horror movies and Grimm's fairy tales will have prepared you for in these situations. You may even become your child's own personal hero for helping them get through these precarious situations. Once everyone is snugly tucked back in their beds, you can all rest easy knowing no matter what, you will get a good night's sleep.